1. Chapter 7
OSI Data Link Layer

2. OSI Data Link Layer
Accessing the Media

3. OSI Data Link Layer Provides the user interface
Segmentation and managing communications
Organize data for network transfer
Exchange data over common media

4. Supporting and Connecting to Upper Layer
Two basic functions:
Allows the upper layers to access the media using framing.
Controls how data is placed on the media and received from the media using media access control (MAC) and error detection.

5. Supporting and Connecting to Upper Layer
Terminology

6. Supporting and Connecting to Upper Layer
Protocol governs how to format a frame for use on that media
Different protocols for different media
Accept the frame Decapsulate to a packet Construct a new frame for the media Forward the new frame

7. Controlling Transfer Across Local Media
Layer 2 protocols specify the encapsulation of a packet into a frame and the techniques for getting the encapsulated packet on and off each media.
The technique is termed the Media Access Control (MAC) method.
Different media might require a different MAC method.
Each media type encountered can have different characteristics. (Especially a router where several different media types can co-exist.)

8. Controlling Transfer Across Local Media
WAN Header
WAN Trailer
LAN Header
LAN Header
LAN Trailer
Packet
Packet
LAN Trailer
Different media…
Different characteristics…
Different MAC method…

9. Controlling Transfer Across Local Media
The protocol can be configured on the device and determines the type of encapsulation (MAC method).
Different media…
Different characteristics…
Different MAC method…

10. Creating a Frame
The description of a frame is the key element of each protocol.
Different protocols require different information to function properly.
Which nodes are in communication with each other.
When communication between individual nodes begins and when it ends.
Which errors occurred while the nodes communicated.
Which nodes will communicate next.

11. Creating a Frame Frames travel the media as a stream of bits.
The framing of the packet inserts control information in specific fields……MORE DETAILS LATER…..

12. Connecting Upper-Layer Services to the Media
Network devices have both a hardware and software component.

13. Connecting Upper-Layer Services to the Media
Data Link Layer prepares the packets from the upper layer software processes for transmission over the physical media.

14. Standards Not defined by RFCs as in the other layers.
Defined by engineering organizations.

15. Connecting Upper-Layer Services to the Media
IEEE
Logical Link Control (LLC) Sub-Layer
Frames the network layer packet
Identifies the network layer protocol
Media Access Control (MAC) Sub-Layer
Addresses the frame
Marks the beginning and end of the frame

16. Placing Data on the Media
Regulating the placement of data on the media is termed Media Access Control.
Media sharing: If and how the nodes share the media.
Topology: How the connection appears to the Data Link Layer.

17. MAC for Shared Media
Two Basic Methods:
More in Chapter 9

18. MAC for Non-Shared Media
Point-to-Point Connections: Type of communication.
Half Duplex
Full Duplex

19. Logical Topology vs Physical Topology
Arrangement of the nodes and the physical connections between them (More in Chapter 9).
Logical Topology:
The way a network transfers frames among nodes. The use of virtual connections between the nodes regardless of the actual physical connection.
Point-to-Point
Multiaccess
Ring

20. Point-to-Point Logical Topology
Concerned with full or half duplex.

21. Multiaccess Logical Topology
Enables a number of nodes to communicate using the same shared media.
A sends to E
Check for other transmissions
Media available
Transmit X X X
I’ll wait……
Media NOT available
Check for other transmissions
B needs to send to D

22. Ring Logical Topology
Each node receives a frame in turn. If it is not addressed to the node, it passes it on.
Yes
Is it for me? No
Is it for me?
A sends to D No
Is it for me?

23. Addressing and Framing Data
OSI Data Link Layer
Addressing and Framing Data

24. The Frame There are many different Data Link Layer protocols….
The Data Link Layer protocol describes the features required for the transport of frames.
Integrated into the encapsulation process.
No single frame protocol meets the needs of all data transportation across all types of media.

25. The Frame Header Data Trailer
However, each Data Link Layer protocol is constructed using the same basic format.
It’s the contents that differ….
Header
Data
Trailer

26. Framing: Role of the Header
Contains the control information required by the protocol.
Some sample fields……
Start of Frame
Source and Destination MAC Addresses
Priority/Quality of Service
Type/Length
Logical Connection Control
Physical Link Control
Flow Control
Congestion Control

27. Framing: Role of the Header
Specific bit sequence that indicates to the receiving device that the frame starts here.
For example:
Header
Data
Trailer
Start
Address
Type/Length
The source and destination MAC addresses.
A code identifying the type of frame OR the total length of the frame.

28. Addressing: Where the Frame Goes
The addresses used this layer are referred to as physical addresses.
They are the burned-in MAC addresses of the network device (PC: NIC, Router: Physical port).
They are only used for local delivery.
If the frame must be transported to another segment, the frame is re-encapsulated by the receiving device and forwarded.
Header
Data
Trailer
Start
Address
Type/Length

29. Addressing: Where the Frame Goes
OR send as a broadcast
The Data Link Layer protocol will define the addressing required to move the frame through the network.

30. Framing: Role of the Trailer
Frame Check Sequence (FCS): Used to provide basic error checking, usually with a Cyclic Redundancy Check (CRC). - Sending device uses an algorithm on the bits of the header and data portions. - The resulting value is placed in the FCS field. - Receiving device does the same thing. - If the values match – no error. - If the values do not match – frame discarded.
Header
Data
Trailer
FCS
Stop
A specific sequence of bits that indicate the end of the frame.

31. Sample: Data Link Layer Frames
Point-to-Point Protocol (PPP)
High Level Data Link Control Protocol (HDLC)
Frame Relay Protocol
Ethernet Protocol

32. Ethernet Protocol for LANs

33. Point-to-Point Protocol for WANs

34. Wireless Protocol for LANs